The present invention relates to a brake control apparatus for motor vehicles which are used for preventing the wheels from being locked up and slipping.
When an excessive torque is applied to the driving wheels due to acceleration, conditions of road surface and the like, a critical slip occurs between the wheels and the road surface. It is likely to cause engine torque to fail to be effectively transmitted to the road surface. In addition, when rapidly changing the steering operation on turning, the vehicle tends to fail in retaining a stable attitude to spin. Moreover, when being suddenly braked or braked on a frozen road surface, the wheels undergo an excessive braking force and thus tend to be locked up. If the wheels are locked under such braking condition, it is greatly difficult to control the vehicle.
Various types of brake control apparatus have been proposed for the purposes of restraining wheel slip, wheel lockup while being braked, and vehicle spin upon rapid change of steering.
For instance, Japanese Patent Application First Publication 4-231255 proposes a brake control device including a main pump and an auxiliary pump which are arranged in series for supplying a brake fluid pressure to wheel cylinders of the driving and driven wheels. The auxiliary pump is connected to a master cylinder so as to deliver the brake fluid within the master cylinder to the main pump. A brake fluid return passage is connected at one end to the wheel cylinders and at the other end to a fluid passage provided for fluid communication between the main and auxiliary pumps. A check valve is disposed in the return passage to prevent a brake fluid flow from the auxiliary pump to the wheel cylinders through the return passage. A reservoir tank is disposed in the return passage between the check valve and the wheel cylinders in order to temporarily store brake fluid fed from the wheel cylinders. The main and auxiliary pumps are actuated to feed brake fluid within the master cylinder to the wheel cylinders in response to the occurrence of wheel slip.
However, since in the brake control device of such conventional type, the check valve is arranged within the return passage, the device must be provided with a mounting site for the check valve within a casing of the device. This leads to a complicated structure of the device with increased size and weight of the casing, resulting in deteriorated installation and cost performance of the device.
U.S. Pat. No. 5,015,043 proposes a brake control device including a main pump and an auxiliary pump which are actuated by individual motors.
With provision of the individual motors, the device of such conventional type increases power consumption and has deteriorated installation and cost performance by using a hydraulic unit having an enlarged size.
In view of disadvantages of the conventional devices as described above, there is a demand for a brake control device having an improved installation and cost performance without failing an operating performance.
Moreover, there is further demand for a pump unit for use in a brake control device which includes main and auxiliary pumps operable by a single motor having a relatively small capacity and capable of performing smooth and quick pumping motion with reducing pulsation, serving for improving a quick responsiveness to various operating condition of the vehicle.